Method of reducing interference, and radio system

ABSTRACT

The invention relates to a method of reducing interference in a radio system comprising at least one base station and a subscriber terminal. The data to be transmitted is converted into symbols in a transmitter and each symbol is multiplied by a connection-specific spreading code composed of chips. The data to be transmitted on connections in a synchronizing phase is multiplied by a spreading code whose duration in time differs from the spreading code in the other connections.

FIELD OF THE INVENTION

The invention relates to a method of reducing interference in a radiosystem comprising at least one base station and a subscriber terminalincluding a receiver and a transmitter in connection with one another,in which transmitter the data to be transmitted is converted intosymbols and each symbol is multiplied by a connection-specific spreadingcode composed of chips.

The invention also relates to a radio system comprising at least onebase station and subscriber terminals comprising a receiver and atransmitter arranged to multiply the data to be sent by aconnection-specific spreading code, which data comprises symbols andwhich spreading code comprises chips.

DESCRIPTION OF THE PRIOR ART

In CDMA (Code Division Multiple Access), a narrow-band signal of a useris multiplied to a significantly broader band by a spreading code. Theusers transmit simultaneously in the same frequency band. On eachconnection between the base station and a subscriber terminal, adifferent spreading code is used, and the signals of the users can beseparated from one another in the receivers on the basis of thespreading code of each user. The spreading codes are chosen in such away as to correlate with one another as little as possible. The signalsmultiplied by another spreading code do not correlate in an ideal caseand do not restore to the narrow bandwidth, but appear as noise.

In a conventional CDMA, however, cross-correlations between differentspreading codes are typically too high, since duration in time andusually also the length in chips of the mutually corresponding spreadingcodes is the same in different connections. This is a problem especiallyduring a synchronizing phase which becomes more difficult because of themutual interference between the users

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to implement a method and a radiosystem wherein a cross-correlation between spreading codes is reducedduring a synchronizing phase, whereby synchronization becomes easier andit is also possible to reduce interference.

This object is achieved by the method introduced in the preamblecharacterized by multiplying the data to be transmitted of at least oneconnection in the synchronizing phase by the spreading code whoseduration in time differs from the corresponding spreading code in otherconnections by some chips at the most in such a way that thecross-correlation of the spreading codes receives different values atdifferent periods of time.

A radio system of the invention is characterized in that it is arrangedto employ a spreading code in at least one connection in thesynchronizing phase, the duration in time of the spreading codediffering by some chips at the most from the corresponding spreadingcode in other connections in such a way that the cross-correlation ofthe spreading codes receives different values at different periods oftime.

The method of the invention provides many advantages in a radio system.The cross-correlation of the spreading codes can be substantiallyreduced in the synchronizing phase, whereby synchronization is improvedand interference is also reduced.

DESCRIPTION OF THE DRAWINGS

In the following, the invention is described in more detail withreference to the examples illustrated in the attached drawings, in which

FIG. 1 illustrates a radio system,

FIG. 2 illustrates a block diagram of a transceiver,

FIG. 3 illustrates cross-correlation between two spreading codes,

FIG. 4 illustrates cross-correlation between two spreading codes,

FIG. 5 illustrates spread-coded data at a fixed chip frequency and

FIG. 6 illustrates spread-coded data at a fixed symbol frequency.

DETAILED DESCRIPTION OF THE INVENTION

The invention can be applied in any radio system employing a spreadingcode, for example in the CDMA system.

FIG. 1 shows a typical CDMA radio system. The radio system comprisescells, each cell typically including one base station 1 and 2 and anumber of subscriber terminals 3 to 5, preferably mobile phones. Boththe base station 1 and 2 and the subscriber terminal 3 to 5 comprises atleast one transceiver through which connections 6 between the subscriberterminals 3 to 5 and the base stations 1 and 2 are established andmaintained. The subscriber terminals 3 to 5, preferably mobile phones,and the base stations 1 and

FIG. 2 shows a block diagram of a typical transceiver including areceiver 10 and a transmitter 11. The transceiver comprises an antenna20, radio frequency means 21, a transmitter multiplier 23, coding means22, a demodulator 24, a modulator 25, control means 26 and a receivermultiplier 27. Digital data, for example synchronizing-related data suchas Sync Channel Message to be transmitted in the transmitter ismodulated in the modulator 25. The data can be processed in themodulator 25 in many ways for example by means of convolution coding andinterleaving. After the modulator 25, the signal to be transmitted ismultiplied to a broadband in the multiplier 23 by a spreading codearriving from the coding means 22. The broadband signal formed hereby isconverted into radio frequency in the radio frequency means 21 whereinthe broadband digital signal is typically multiplied by a high-frequencyanalog signal of a local oscillator and is then high-pass filtered. Theradio frequency signal obtained hereby is transmitted by means of theantenna.

FIG. 2 shows a block diagram of a typical transceiver including areceiver 10 and a transmitter 11. The transceiver comprises an antenna20, radio frequency means 21, a transmitter multiplier 23, coding means22, a demodulator 24, a modulator 25, control means 26 and a receivermultiplier 27. Digital data, for example synchronizing-related data suchas Sync Channel Message to be transmitted in the transmitter ismodulated in the modulator 25. The data can be processed in themodulator 25 in many ways for example by means of convolution coding andinterleaving. After the modulator 25, the signal to be transmitted ismultiplied to a broadband in the multiplier 23 by a spreading codearriving from the coding means 22. The broadband signal formed hereby isconverted into radio frequency in the radio frequency means 21 whereinthe broadband digital signal is typically multiplied by a high-frequencyanalog signal of a local oscillator and is then high-pass filtered. Theradio frequency signal obtained hereby is transmitted by means of theantenna.

At the receiver part 10, the antenna 20 receives the signal propagatingto the radio frequency means 21 wherein the signal is typicallymultiplied by a high-frequency analog signal of a local oscillator andis then low-pass filtered. The remaining wide-band signal is digitizedand multiplied in the multiplier 27 by the spreading code of thereceiver, the spreading code arriving from the coding means 22. In thepresent block diagram, the multiplier 27 also comprises a correlatorwherein the code phase is being searched. The output of the multiplier27 thus includes a narrow-band signal containing digital information,the signal being demodulated in the demodulator 24 by means of forexample deconvolution coding and de-interleaving. The transceiveroperation is typically controlled by the control means 26.

Let us have a closer look at the spreading code searching process when aCDMA receiver is being synchronized. The synchronizing channel isdirected from the base station to the subscriber terminal and it is usedfor synchronizing the connection between the subscriber terminal and thebase station. Once the subscriber terminal is synchronized to the radiosystem, the subscriber terminal does not usually re-use thesynchronizing channel. A synchronizing-channel frame is as long as thepseudo-random sequence of a pilot signal, and the frame is transmittedsimultaneously with the pilot sequence. Since each base station hasdifferent pilot sequence time offsets, timings of the frames of thesynchronizing channels also differ in each base station. Thus,synchronizing to the pseudo-random sequence of the pilot signalfacilitates the synchronization of the subscriber terminal to thesynchronizing channel. Synchronization is difficult because thecross-correlations between different spreading codes are typically toohigh and because the mutually corresponding spreading codes have thesame duration in time and usually also the length in chips in differentconnections.

Let us assume that the receiver attempts to find a desired spreadingcode of a particular user from the group of spreading codes of K users.The search is carried out by forming a correlation between the receivedsignal and the desired spreading code derived from the local codingmeans 22. The correlation is obtained for example from $\begin{matrix}{{{C\quad (n)} = {\sum\limits_{k = 1}^{N}\quad {S_{p}\quad (k)*S_{r}\quad \left( {n - k} \right)}}},} & (1)\end{matrix}$

where S_(p) and S_(r) are the spreading codes, n is the sample, k is thephase delay and N stands for the code length. If in the formula (1) thecorrelation subindex p is r, i.e. p=r, auto correlation is involved. Incontrast, if in the formula (1) p is not r, i.e. p≠r, cross-correlationis involved. The output signal y(t) of the correlator 27 during one datasymbol is $\begin{matrix}{{{y\quad (t)} = {{x_{0}\quad (t)} + {\sum\limits_{k = 1}^{N - 1}\quad {x_{k}\quad (t)}} + {n\quad (t)}}},} & (2)\end{matrix}$

where x₀(t) is the autocorrelation result of the spreading code signalto be searched, the sum term is the cross-correlation result of theother spreading code signals and n(t) is stands for the noise.

Let us study the method of the invention in more detail. At least in oneconnection 6 in the synchronizing phase between the subscriber terminal3 to 5 and the base station 1 and 2, the connection being typically aunidirectional connection from the base station to the subscriberterminal, the data to be transmitted is multiplied by the spreading codewhose duration in time or the length in chips differs from thecorresponding spreading code in the other connections. A spreading codeof a different length is not preferably employed in every connection inthe synchronizing phase, but the radio system also comprises connectionsemploying the spreading code of the same length, the connections forminga group of similar connections. The radio system comprises, however, anumber of connection groups wherein the data in the synchronizing phaseis multiplied by a spreading code deviating from the correspondingspreading code in the other groups. If one connection in the radiosystem employs a number of spreading codes affecting the signal atdifferent levels or in different ways, the corresponding spreading codeis taken to mean the spreading code at the same level or the spreadingcode affecting the signal in the same way.

The signal can be transmitted both at a chip frequency and at a symbolfrequency on different connections in the synchronizing phase. Thus, thespreading code correlation of different connections is as low aspossible. The duration in time of the spreading code can be preferablychanged for each symbol in each connection. Thus, each symbol to betransmitted is preferably multiplied by a spreading code whose durationin time differs from the corresponding spreading code in the otherconnections. The duration in time of the spreading codes can differ fromone another preferably by one or more chips.

A fixed chip frequency can also be employed as a spreading code chipfrequency, whereby the symbol frequency changes according to theduration in time of the spreading code and particularly in this caseaccording to the chip length. When the fixed symbol frequency is used,the correlation between the spreading codes is reduced by employing avariable chip frequency and the duration in time of the spreading codeon different connections. The receiver forms a correlation between thereceived signal and the spreading code of the signal to be detected andthe correlation is averaged over several symbols, whereby the effect ofthe other spreading codes on the signal to be detected is reduced andthe signal can be detected better.

Let us study the solution of the invention in more detail by means ofFIGS. 3 to 6. FIG. 3 shows a typical autocorrelation graph ac of aspreading code, a cross-correlation graph cc of any two spreading codesand a noise graph n. The autocorrelation function ac and thecross-correlation function cc of the spreading codes remain the samefrom one symbol to the other. The autocorrelation peak is at point 30.Different spreading codes correlate slightly more than on average atpoints 31. Correlation peaks caused by the noise are random and theirintensity and location differ from one symbol to the other. The noiseeffect on the output signal of the correlator y(t) can be reduced byaveraging the correlation result over several symbols. However, thisdoes not lower or shift the maximum points of the cross-correlation indifferent spreading codes, since the spreading codes are of the samelength and alike for all the symbols.

When the spreading codes of different users are of different lengths,the cross-correlation between the spreading codes varies from one symbolto the other. The duration of the first spreading code, i.e. in thiscase the length is N₀ and the length of the second spreading code isN₁=N₀±ΔN, where N₀ and N₁ stand for the number of chips and ΔNrepresents one spreading code chip or several spreading code chips.Averaging the cross-correlation result over a number of chips causes themaximum points of the cross-correlation result to be reduced, since thecross-correlation results are different at different periods of time dueto the different lengths of the spreading codes. A difference of even achip or a fraction of a chip affects the cross-correlation resultdramatically, but the difference in length and/or the duration of thespreading codes can differ from one another without a definitequantitative limit. However, a fraction of a chip, one chip or somechips can change the cross-correlation significantly. In this case, someat the most refers to a small quantity in proportion to the length ofthe spreading code, typically for example about 10% of the total lengthof the spreading code. FIG. 4 shows the auto-correlation graph ac of thespreading code, the cross-correlation graph cc of any two spreadingcodes and the noise graph n of the invention averaged over a number ofsymbols. FIG. 4 shows that only the autocorrelation peak 30substantially remains.

FIG. 5 illustrates the operation of the inventive idea when the chipfrequency of the spreading code is kept constant. Thus, the duration ofa data symbol differs in different spreading codes. FIG. 5 comprisesdata symbols 40 and spreading code chips 41. In FIG. 5, the length ofthe spreading codes differs by one chip, whereby the spreading codes asif slide or shift by one chip with regard to one another from one symbolto the other. A corresponding shift also takes place in the other lengthdifferences of the spreading codes. The shift changes thecross-correlation of the spreading codes in different symbols, wherebythe cross-correlation is reduced when averaged over a number of symbols.

FIG. 6 illustrates the operation of the inventive idea when the chipfrequency is not constant. FIG. 6 comprises data symbols 40 andspreading code chips 41. When the chip frequency and also the durationin time or the length are not constant, an efficient shift of the chips41 and the symbols 40 with regard to one another is obtained, wherebythe cross-correlation of the spreading codes is further reduced.

The inventive solution can be implemented for example by using aspreading code bank available to the base station, the spreading codebank containing a group of predetermined spreading codes from which thebase station employs a new code whenever it sends a signal required forsynchronization in the establishment of a new connection. The group ofspreading signals in the spreading signal bank is preferablypredetermined. The base station decides upon the way of synchronization.A predetermined time can be employed for the synchronization. If thesynchronization fails, the connection must be re-established.Alternatively, the base station can send the signal required forsynchronization as long as the subscriber terminal is definitely able tobe synchronized to it.

The solutions of the invention can be implemented particularly fordigital signal processing by means of for example ASIC or VLSI(Application-Specific Integrated Circuit, Very Large Scale Integration).The operations to be accomplished are preferably implemented as programsbased on microprocessor technique.

Although the invention is described above with reference to the exampleillustrated in the attached drawings, it is to be understood that theinvention is not restricted thereto but can be modified in many wayswithin the scope of the inventive idea presented in the attached claims.

What is claimed is:
 1. A method of reducing interference in a radiosystem comprising at least one base station and a subscriber terminalincluding a receiver and a transmitter in connection with one another,in the transmitter the data to be transmitted is converted into symbolsand each symbol is multiplied by a connection-specific spreading codecomposed of chips, the method comprising the step: multiplying data ofat least one connection in a synchronizing phase by a spreading codewhose duration in time differs from the corresponding spreading code inother connections by some chips at the most in such a way that thecross-correlation of the spreading codes receives different values atdifferent periods of time; and transmitting the data on differentconnections in the synchronizing phase by employing different chipfrequency and symbol frequency.
 2. A method as claimed in claim 1,wherein the multiplying step comprises multiplying data to betransmitted in part of the connections in the synchronizing phase by thespreading code whose duration in time differs from the correspondingspreading code in the other connections.
 3. A method as claimed in claim1, wherein the multiplying step comprises multiplying each symbol in thedata to be transmitted in the synchronizing phase by the spreading codewhose duration in time differs from the corresponding spreading code inthe other connections.
 4. A method as claimed in claim 1, wherein themultiplying step comprises durations in time of different spreadingcodes in the synchronizing phase which differ from one another by one ormore chips.
 5. A method as claimed in claim 1, further comprisingemploying a fixed chip frequency as a chip frequency for differentspreading codes in the synchronizing phase.
 6. A method as claimed inclaim 1, further comprising employing a fixed symbol frequency as a datasymbol frequency in the synchronizing phase.
 7. A method as claimed inclaim 1, further comprising forming a correlation in the receiver in thesynchronizing phase between a received signal and the spreading code ofa signal to be detected and averaging the correlation over severalsymbols whereby the effect of the other spreading codes on the spreadingcode of the signal to be detected is reduced.
 8. A method as claimed inclaim 1, further comprising, at the base station, selecting a spreadingcode from a predetermined group of spreading codes for each connectionin the synchronizing phase.
 9. A radio system comprising at least onebase station and subscriber terminals comprising a receiver and atransmitter arranged to multiply data to be sent by aconnection-specific spreading code, which data comprises symbols andwhich spreading code comprises chips, wherein the radio system isarranged to employ the spreading code in at least one connection in thesynchronizing phase, the duration in time of the spreading codediffering by some chips at the most from the corresponding spreadingcode in other connections in such a way that the cross-correlation ofthe spreading codes receives different values at different periods oftime; and in the synchronizing phase, the radio system is arranged totransmit the signal on different connections at a different chipfrequency and symbol frequency.
 10. A radio system as claimed in claim9, wherein the radio system is arranged to employ the spreading code inpart of the connections in the synchronizing phases, the duration intime of the spreading code differing from the corresponding spreadingcode in the other connections.
 11. A radio system as claimed in claim 9,wherein the radio system is arranged to multiply each symbol to betransmitted in the connection in the synchronizing phase by a spreadingcode whose duration in time differs from the corresponding spreadingcode in the other connections.
 12. A radio system as claimed in claim 9,wherein the duration time of the different spreading codes in thesynchronizing phase differ by one or more chips.
 13. A radio system asclaimed in claim 9, wherein the chip frequency in different spreadingcodes is fixed in the synchronizing phase.
 14. A radio system as claimedin claim 9, wherein the data symbol frequency is fixed in thesynchronizing phase.
 15. A radio system as claimed in claim 9, wherein,in the synchronizing phase, the receiver is arranged to form acorrelation between a received signal and the spreading code of a signalto be detected and to average the correlation over a number of symbolswhereby the effect of the other spreading codes on the spreading code ofthe signal to be detected is reduced.
 16. A radio system as claimed inclaim 9, wherein the base station is arranged to select a spreading codefor the connection in the synchronizing phase from the predeterminedgroup of spreading codes.